1. Field of the Invention
The present invention pertains to marine seismic surveying, and, more particularly, to particle vector measurement in a marine seismic survey.
2. Description of the Related Art
In one kind of marine seismic survey, a survey vessel tows an array of seismic cables, frequently referred to as “streamers,” along a predetermined course. As the vessel tows the array, a seismic source, such as an airgun or a vibroseis source, imparts an acoustic wave into the water. The acoustic wave travels through the water and is eventually reflected by various geological features. The reflections travel back up through the water to the streamers. The streamers include acoustic sensors, or “hydrophones,” distributed along its length. As the reflections pass over the acoustic receivers, the receivers sense the magnitude of the passing wavefront. The acoustic receivers then transmit data representing the detected magnitude of the passing wavefront back up the seismic cables to the survey vessel for collection.
The reflections continue propagating through the water past the acoustic receivers until they reach the water's surface. At the surface, the reflections are reflected once again. These reflections of the reflections are sometimes called “multiples,” short for multiple reflections, or “ghost reflections.” The multiples travel back down through the water and will also pass over the acoustic receivers. The acoustic receivers once again sense the magnitude of the passing wavefront. The acoustic receivers also once again transmit data representing the detected magnitude over the seismic cables for collection aboard the survey vessel.
Thus, the survey data contains not only data obtained from the initial reflections, but also data collected from the multiples. The data from the multiples is undesirable because it is not representative of the geological formations being surveyed. Instead, data from the multiples is representative of the surface. More technically, the multiples “destructively interfere” with the reflections. In short, the seismic sensors sense the magnitude of any passing wavefront without regard to the direction of its travel.
Conventional approaches address this problem in two ways. One way is to try and mitigate the influence of multiples during the survey. A second way is to try to back out the multiples data during processing. Both approaches have their drawbacks.
Mitigating multiples during the survey frequently involves positioning the components of the survey in a particular fashion. For example, ghost reflections can often be largely canceled out if the seismic cables are towed at a depth of approximately 4-5 meters. However, positioning streamers can be very difficult. Streamers may be several kilometers long. This typically results in a rather large inertia that can make the streamer difficult to control. The streamer may also be subjected to very different environmental conditions—such as wind and current—along its length. This means that the streamer may frequently be inaccurately positioned so that the adverse effect of the multiples is not fully mitigated.
Backing out the multiples during processing typically involves predicting the actual multiples from a number of factors. A variety of multiples prediction techniques are known to the art. However, as with all prediction techniques, assumptions and generalizations are made. While these generalizations and assumptions may be statistically viable, they may apply to any given survey—or any given portion of a survey—with more or less accuracy. In some surveys, they consequently may have a harmful effect or otherwise create inaccuracies. Furthermore, this approach lengthens complicated processing, thereby driving up costs. It would therefore be desirable to mitigate the effect of multiples without having to expend the time, effort and resources to continuously monitor and position the seismic cables. It would also be desirable to be able to mitigate the efforts of multiples through actual measurements rather than predictions. Consequently, it would be desirable to not measure the magnitude of any given wavefront passing the acoustic sensors, but also its vector, or polarization.                The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.        